Ammonia is an irritant gas and even at sub-ppm concentrations has been found to be associated with respiratory disease. Occupational exposures to ammonia are encountered in commercial refrigeration, janitorial work and animal agriculture. Occupational exposure to ammonia have been linked to adverse health outcomes at levels well below the current PEL of 50 ppm, however understanding the specific role that chronic exposures to low levels of ammonia play in these adverse health outcomes is hampered by an almost complete absence of data on personal occupational exposures to ammonia at low-ppm and sub-ppm concentrations. This data gap exists in large part because current ammonia monitoring equipment is expensive, is often not portable, and consequently is poorly suited to personal and community exposure monitoring. The objective of this proposed project is to develop a low cost ammonia sensor array that can be integrated with sensors for particle mass and other gases, for use in exposure assessments of ammonia concentrations in workplaces such as animal feeding operations. The specific aims of the project are: 1) Fabricate sub- millimeter scale ammonia sensors with sub-ppm sensitivity based on reduced graphene oxide chemiresistors; 2) Optimize the reduced graphene oxide sensor array for selectivity and sensitivity; 3) Incorporate the new ammonia sensor element into an existing portable monitoring device; and 4) Evaluate performance of the portable monitoring device in the field. Our proposed research addresses several aspects of NIOSH' National Occupational Research Agenda (NORA). The novel ammonia sensor, incorporated into a portable exposure monitoring device addresses health risks in the Agriculture, Forestry and Fishing Sector, and the Services Sector, and cross sector programs in Exposure Assessment, Respiratory Diseases and Total Worker Health. The tools developed in this proposal are not specific to ammonia sensors but are broadly intertwined with efforts in advancing the technology to use low cost sensors for exposure measurements. The outputs from this project are 1) assessments of novel ammonia sensor technologies for use in occupational hygiene 2) development of a new ammonia monitor with sub-ppm sensitivity for continuous ammonia monitoring 3) novel designs for adapting the low cost sensors for use in a personal exposure monitor 4) improved informatics tools for processing and analyzing data applicable to a low cost monitoring network.